A liquid crystal display generally comprises at least a liquid crystal cell, a polarizing plate and an optical compensating sheet (phase retarder plate). Concretely, a transmission liquid crystal display comprises two polarizing plates disposed on both sides of a liquid crystal cell therein, and one or two optical compensating sheets sandwiched between the liquid crystal cell and the polarizing plate. A reflection liquid crystal display comprises a reflector, a liquid crystal cell, one optical compensating sheet and one polarizing plate that are arrayed in that order.
The liquid crystal cell in such displays generally comprises rod liquid-crystalline molecules, two substrates for sealing up them, and an electrode layer for applying voltage to the rod liquid-crystalline molecules. Various display modes of liquid crystal cells are proposed, depending on the difference in orientation of the rod liquid-crystalline molecules in the cells. For example, TN (twisted nematic)-mode cells, IPS (in-plane switching)-mode cells, FLC (ferroelectric liquid crystal)-mode cells, OCB (optically compensatory bend)-mode cells, STN (super twisted nematic)-mode cells and VA (vertically aligned)-mode cells are for transmission displays; and HAN (hybrid aligned nematic)-mode cells are for reflection displays.
The polarizing plate generally comprises a polarizing film and a transparent protective film, and the polarizing film is generally prepared by dipping a polyvinyl alcohol film in an aqueous solution of iodine or dichromatic dye followed by monoaxially stretching the thus-dipped film. Two transparent protective film are attached to both sides of the polarizing film to construct the polarizing plate.
Optical compensating sheets are used in various liquid crystal displays for solving a problem of image discoloration and for enlarging the field of view.
Of those, λ/4 plates have many applications, for example, for optical compensating films for liquid crystal displays, and antireflection films for organic EL displays, and they are now in practical use. However, many λ/4 plates heretofore used in the art attain λ/4 or λ/2 only in a specific wavelength range.
JP-A 27118/1993 and 27119/1993 disclose an optical compensating film fabricated by laminating a birefringent film of large retardation and a birefringent film of small retardation in such a manner that their optical axes cross each other at right angles. The compensating film could theoretically function as a λ/4 plate in the overall range of visible light, so far as the difference in retardation between the laminated two films is λ/4 in the overall range of visible light. JP-A 68816/1998 discloses an optical compensating film capable of attaining λ/4 in a broad wavelength range, which is fabricated by laminating a polymer film of λ/4 in a specific wavelength range and a polymer film of λ/2 of the same material in the same wavelength range as that of the former. JP-A 90521/1998 also discloses an optical compensating film fabricated by laminating two polymer films and capable of attaining λ/4 in a broad wavelength range. However, the optical compensating film of the type fabricated by laminating two films has various problems in that it is thick and its cost is high. Therefore, an optical compensating film of a single film that realizes λ/4 in a broad wavelength range is desired.
In this connection, JP-A 2000-137116 and WO 00/65384 have a description relating to an optical compensating film of a single polymer film of which the phase retarder reduces in a shorter wavelength range, and to its application to circularly polarizing plates and reflection liquid crystal displays. As the parameter of controlling the view angle characteristic of the above-mentioned λ/4 plate, employed is a numerical value defined by (nx−nz)/(nx−ny) (this is hereinafter referred to as an NZ factor). nx, ny and nz indicate the refractive index along the slow axis in plain (the maximum refractive index in plain) of the phase retarder, the refractive index perpendicular to the slow axis in plane of the phase retarder, and the refractive index along the thickness direction, respectively. WO 00/65384 says that the preferred range of the NZ factor is 1≦NZ≦2.
Preferably, the NZ factor is controllable. This is because, in a liquid crystal display for image formation, the birefringence (Δn) of the liquid crystal cell varies depending on the liquid crystal panel therein and the angle-dependency of Δn also varies depending on the liquid crystal panel. Therefore, if the NZ factor of the optical compensating film in the display is controllable, the view angle characteristic of the display can be optimized, requiring no change of the retardation value Re of the film.
However, the NZ factor is defined by the refractive indices in three directions of the film, and is therefore correlated with the draw ratio of the film. Concretely, when the draw ratio of the film in the machine direction increases more and therefore the film is most likely in monoaxial orientation, then the NZ factor of the film comes nearer to 1 from a larger value. In case where a λ/4 plate is fabricated according to the width-unlimited monoaxial stretching method described in the Examples in WO 00/65384, the draw ratio of the film that realizes a retardation of λ/4 is determined by the elongation at break of the film, and therefore the NZ factor of the film shall be indiscriminately determined.